Stereophonic sound works on the principle that differences in sound heard between the two ears by a human get processed by the brain to give distance and direction to the sound. To exploit this effect, reproduction systems use recorded audio signals in left and right channels, which correspond to the sound to be heard by the left ear and the right ear, respectively. When the listener is wearing headphones, the left channel sound is directed to the listener's left ear and the right channel sound is directed to the listener's right ear. However, when sound is produced by a pair of speakers, sound from a left channel speaker can be heard by the listener's right ear and sound from a right channel speaker can be heard by the listener's left ear. When the listener moves relative to the location of the speakers the depth of feeling of the reproduced sound will change. Stereo speaker systems typically rely on the physical separation between the left and right speakers to produce stereophonic sound, but the result is often a sound that appears in front of the listener. Modern sound systems include additional speakers to surround the listener so that the sound appears to originate from all around the listener.